I once had a netgear router that I applied firmware updates to. It would corrupt any file that was downloaded over 2gb. They had another firmware update within a couple weeks but it was a hard problem to track down. Just throwing this out there so that if you have any firmware updates for your router, it might be a good idea to run them.
It might be a good idea to go to the event log and look for disk errors just in case your hard drive is failing. Normally you can get there by Start > Run > type "eventvwr". Look under the system section and scroll down through the messages looking for "disk" in the "source" column.
It should look something like this
There is a pretty common disk error that mentions the page file. You can ignore those messages but if you have a ton of disk errors in a row like the screenshots then it's possible you have a hard drive failing.